Forscan Tire Size Calculator

Convert tire dimensions into hex values for speedometer calibration

Old (Stock) Tire Size

New Tire Size

What is a FORScan Tire Size Calculator?

A forscan tire size calculator is a specialized tool used by Ford, Lincoln, and Mazda enthusiasts to recalibrate their vehicle’s computer systems after changing tire dimensions. When you upgrade to larger off-road tires or swap to different street wheels, your vehicle’s factory-programmed tire circumference no longer matches the physical reality. This discrepancy leads to inaccurate speedometer readings, incorrect odometer logging, and can even affect transmission shift points in modern trucks like the Ford F-150 or Ranger.

Who should use it? Any vehicle owner utilizing the FORScan software to modify their Body Control Module (BCM) or Powertrain Control Module (PCM). Common misconceptions include the belief that the truck will “learn” the new size automatically. In reality, modern Ford vehicles rely on a specific four-digit hexadecimal code representing the tire circumference in millimeters to calculate vehicle speed.

FORScan Tire Size Calculator Formula and Mathematical Explanation

The calculation behind a forscan tire size calculator involves multiple steps to convert standard tire sidewall markings into a digital format the BCM understands. The primary goal is to find the rolling circumference in millimeters.

The Step-by-Step Derivation:

1. Calculate Tire Height (Inches): (Width × Aspect Ratio × 2 / 2540) + Rim Diameter.
2. Convert to Millimeters: Height in inches × 25.4.
3. Calculate Circumference: Diameter (mm) × π (3.14159).
4. Convert to Hexadecimal: The circumference (mm) is converted from a base-10 integer to a base-16 hex value.

Variable	Meaning	Unit	Typical Range
Width	Tire section width from sidewall	mm	215 – 315
Aspect Ratio	Height as percentage of width	%	35 – 85
Rim	Wheel diameter	inches	15 – 22
Circumference	Total distance of one rotation	mm	2000 – 3000
Hex Code	FORScan BCM entry value	Hex	0700 – 0BFF

Practical Examples (Real-World Use Cases)

Using the forscan tire size calculator provides precision that “guesstimating” cannot achieve. Let’s look at two common Ford F-150 scenarios.

Example 1: The Leveling Kit Upgrade

A user moves from factory 275/65R18 tires to 285/75R18 tires.
Inputs: Old (275, 65, 18), New (285, 75, 18).
Output: The circumference jumps from roughly 2559mm to 2773mm.
FORScan Impact: The hex code changes from 09FF to 0AD5. Without this update, at an indicated 60 mph, the truck would actually be traveling at 65 mph, increasing the risk of speeding tickets and excessive wear.

Example 2: Street Performance Swap

An owner swaps heavy stock wheels for 305/40R22 performance tires.
Inputs: Old (275/55R20), New (305/40R22).
Interpretation: Even though the rim is larger, the total diameter change is only +0.8%. The forscan tire size calculator shows a hex value change of only a few digits, suggesting that while the speedometer is close, a minor adjustment is still needed for perfect odometer tracking.

How to Use This FORScan Tire Size Calculator

To get the most accurate results for your Ford’s BCM, follow these steps:

• Gather Data: Locate the three numbers on your current tire sidewall (e.g., 275/70R18).
• Input Stock Values: Enter your original factory tire dimensions into the “Old Tire Size” section.
• Input New Values: Enter the dimensions of the tires you have installed or plan to install.
• Read the Hex Code: Look at the large green result box. This is your 4-digit hex value.
• Apply in FORScan: Connect your OBDII adapter, open FORScan, go to BCM Configuration, and find address 726-12-01. Replace the first four digits of the existing hex with your new result.
• Relearn PCM: After updating the BCM, perform a “PCM Module Tire Size Relearn” to avoid a P160A DTC code.

Key Factors That Affect FORScan Tire Size Results

1. Tread Depth: A brand new tire has a larger circumference than a worn one. The forscan tire size calculator uses theoretical values; if your tires are half-worn, you might need to subtract 5-10mm from the result.
2. Manufacturer Variance: Not all “35-inch” tires are 35 inches. Check the manufacturer spec sheet for the actual “Static Loaded Radius” or “Revs Per Mile.”
3. Tire Pressure: Lower PSI decreases the effective rolling radius, meaning the tire covers less ground per rotation.
4. Vehicle Weight: Heavier trucks compress the tire more, effectively shortening the radius between the axle and the ground.
5. Module Limitations: Some older BCMs have a maximum hex value limit. If your tires are too large (e.g., 40+ inches), the computer may reject the value.
6. GPS Validation: Always verify your forscan tire size calculator results against a GPS speedometer app once programmed.

Frequently Asked Questions (FAQ)

Why is my speedometer still off by 1-2 mph?

Calculated circumference is theoretical. Real-world “squish” under vehicle weight usually makes the tire behave slightly smaller than the math suggests. Adjust your hex value down by 10-20mm if the speedometer reads too high.

What is the P160A error code?

This is a “Control Module Software Parameter Incompatibility” code. It happens when the BCM tire size doesn’t match the PCM. Use the FORScan “Configuration and Programming” tab to run the “Tire Size Relearn” procedure.

Can I use this for non-Ford vehicles?

The physical math (circumference) works for any vehicle, but the specific 4-digit Hex output format is designed specifically for Ford’s BCM architecture.

Do I need to change both BCM and PCM?

Usually, you change the BCM value, then use the FORScan service function to “Sync” or “Relearn” that value into the PCM and TCM.

What is the max tire size FORScan can handle?

Most Ford BCMs stop accepting values around 2850mm to 2900mm. If your tires are larger (like 40s), you may need a specialized inline speedometer calibrator.

Does tire size affect my transmission?

Yes. Larger tires effectively change your final drive ratio. Programming the correct size helps the TCM (Transmission Control Module) maintain proper shift points.

How do I convert revs per mile to hex?

Divide 1,609,344 by your “Revs Per Mile” to get the circumference in mm, then convert that number to hexadecimal.

Is this permanent?

No, you can use the forscan tire size calculator to find your original values and flash them back at any time using your FORScan backup files.

Related Tools and Internal Resources

• Tire Size Comparison Guide – Compare two tire sizes side-by-side with visual overlays.
• Speedometer Correction Tools – Explore different hardware and software options for Ford trucks.
• Ford Performance Tuning – Learn how tire changes impact your 0-60 and towing performance.
• Wheel Offset Calculator – Ensure your new larger tires won’t rub on the suspension.
• Drivetrain Gear Ratio Calculator – Determine if you need to re-gear after installing 35″ or 37″ tires.
• Off-Road Tire Selection – Our guide to picking the right tread pattern for your Ford Ranger or F-150.